Vertebrate mesodermal formation is a highly intricate and invariant process. In the vertebrate limb over 40 muscles must be specified during embryonic development. Very little is known about the molecular factors and pathways responsible for muscle formation. Recently, the PIs have uncovered a role for the Wnt signaling pathway in muscle patterning in the vertebrate limb. Surprisingly, the Wnt pathway is not required within developing muscles, but in a population of cells that surround the developing muscles. The cell population in which the Wnt pathway is active creates a muscle "pre-pattern". Through an unknown mechanism, migrating myoblasts appear to sense the pre-pattern produced by cells containing an active Wnt pathway and begin to differentiate. The most downstream components of the Wnt signaling pathway are the Tcf/Lef class of transcription factors. In the mouse and chick limb the PIs have shown that Tcf4 is expressed in a population of cells that surrounds differentiating muscles ("perimuscular"). Misexpressing a dominant-negative version of TCF4 in the chick perimuscular region resulted in abnormal muscle formation and a malformed limb. There are a number of identified downstream targets of TCF4 (and Wnt signaling in general) but none of these factors appear to be involved in muscle patterning. It would be expensive and time consuming to perform large-scale screens in the mouse system to identify Wnt signaling targets involved in muscle patterning. The lack of genetics in the chick system makes this type of screen impossible. However, the nematode C. elegans is ideally suited for genetic and molecular screens. C. elegans contains a single TCF4 homologue, POP-1. The PIs show that animals with a mutation in pop-1 have defects in mesodermal patterning and they propose to use this phenotype as the basis for a screen to identify additional genes involved in Wnt signaling. This screen has the potential to identify genes that are direct targets of the Wnt pathway, that work in parallel pathways along with the Wnt pathway, or that function independently of the Wnt pathway to pattern mesodermal tissues. The proposed screen is uniquely suited to identify new factors involved in muscle specification and Wnt signaling in both vertebrates and invertebrates. Once candidate genes have been identified the PIs will then use the chick and mouse systems to validate the roles of these factors in patterning vertebrate mesodermal tissues. The PIs have experience working with C. elegans, mice and chick model systems and are positioned to take full advantage of each system.